This invention relates to a Josephson logic gate device which consists of a closed loop of four Josephson junctions and acts a high-speed switching operation by current injection and relates to a logic circuit which is integrated with a lot of such Josephson logic gate devices.
In the field of electronic computers which is constantly striving for improvements in operational speed and performance, the silicon semiconductor devices used in most electronic computers have begun to reach a limit in operating speed, and Josephson devices have recently begun to attract keen attention as one potential novel break-through. The Josephson device exhibits an excellent property of performing a high-speed switching operation with low power dissipation and high sensitivity in accordance with the Josephson effect produced in a superconductive state at very low temperatures. Therefore, the Josephson device has given rise to expectations for materialization of super-high speed computers. This device, in its basic construction, comprises two superconductors joined to each other through the medium of a thin insulating film (Josephson tunneling junction) as typified by the Josephson tunnel junction device. In this construction, when the current supplied to the junction is varied so much as to exceed the critical current, the device is transferred from the zero-voltage state to the voltage state (a switching operation). With this basic construction unaltered, the device's sensitivity which is defined by the slope in the threshold curve of input-output characteristics, is low and its operation is deficient in stability.
With a view to improving the performance of the basic switching device, there have been proposed magnetically controlled devices (inline gate type; e.g., H. H. Zappe "Josephson quantum interference computer devices" IEEE Transactions on Magnetics, Vol. MAG-13, No. 1, pp. 41-47, January 1977). Originating in the discovery that application of an external magnetic field to the Josephson junction causes to lower the critical current value, these devices utilize the phenomenon that transition of the device from the zero-voltage state to the voltage state is caused by the injection of current into a control line to generate a magnetic field in the proximity of the junction. These devices operate with small power dissipation and entail no problem of emission of heat. Therefore, higher degrees of integration than that in the aforementioned semiconductor devices are possible. When these devices are reduced in size in order to increase their switching speed, their sensitivity is observed to drop with the decrease in the device size. This decline in sensitivity may possibly be precluded by injecting a large current into the control line thereby to increase the magnitude of the magnetic field to be generated. This measure inevitably necessitates extra means for enabling the device to withstand the application of such a large current and impedes materialization of desired integration. Besides, in SQUID type magnetically controlled device (described in the above-mentioned reference literature by H. H. Zappe), the sensitivity of the gate is greatly improved. The reduction of the device size is, however, restricted by the inductance which plays an essential role in the switching operation.
On the other hand, a current injection logic (CIL) device (T. R. Gheewala "Josephson logic circuits based on nonlinear current injection in interferometer devices", Applied Physics Letter Vol. 33(8), pp. 781-783, Oct. 15, 1978) in which the transition of the device from the zero-voltage state to the voltage state is achieved by injecting an input signal current directly into the Josephson junction of the SQUID loop has been proposed. Compared with the former magnetically controlled devices wherein the input signal current pass is isolated from the gate current pass, in the CIL device the current passes of the input signal current and the gate current are directly connected with the passive inductance of the device. This device, therefore, has one disadvantage that when the device switches, the input and output signal currents cannot be isolated from each other.
A current-switched Josephson gate in which the isolation between the input signal current and the gate bias current is achieved with two junctions and a resistor, the slope of the threshold curve is low. (T. A. Fulton et al., "A simple high-performance current-switched Josephson gate", Applied Physics Letter, Vol. 34(10), pp. 709-711, May 15, 1979)
As described above, the conventional Josephson device has been unable to simultaneously satisfy the three conditions, i.e., (1) the size reduction of the device which permits integrated circuits in high density, (2) the high sensitivity which produces wide operation margin, and (3) perfect isolation between the input and output signal currents in the device. The three conditions are indispensable to the components of future electric Josephson computer to obtain a stable logic circuit operation.